The invention relates to accelerometers.
Accelerometers are devices that measure acceleration, or changes in a rate of motion. When an elevator starts or stops, several portions of the human body can detect the change in motion and report the change to the brain. Similarly, known accelerometers use different mechanical and electrical techniques to detect changes in motion, and to report the changes to processors. Accelerometers are used in navigational systems, automatic seat belt and air bag triggers, and many other applications.
In known techniques for manufacturing semiconductors, a single crystal of silicon is grown, and then photographic and lithographic techniques are used to etch away unwanted parts of the silicon, and to introduce doping atoms into the silicon to change the electrical properties of the silicon. It is also known to deposit other materials onto the siliconxe2x80x94for example, thin layers of metal may be deposited onto the silicon to serve as conducting wires between different portions of a circuit. The underlying silicon serves as a structural base to provide mechanical support for the metal, while the metal provides the electrical conductivity.
In general, in a first aspect, the invention features an accelerometer. The accelerometer includes a fixed structure, a movable structure, and a resilient coupling. The fixed and movable structures generally lie in a plane. The fixed structure bears a fixed electrode, and the movable structure bears a movable electrode. The resilient coupling is designed to retain the fixed and movable structures in capacitive opposition to each other across a capacitance gap while allowing motion of the movable electrode relative to the fixed electrode in response to acceleration along an axis of acceleration perpendicular to the plane, and to resiliently restore the two electrodes to an equilibrium position when the acceleration ceases. Electronics and/or software is designed to translate a measurement of capacitance between the fixed and movable electrodes into a measurement of the acceleration along the axis.
In general, in a second aspect, the invention features an accelerometer. The accelerometer includes a fixed portion, a movable portion, and a resilient coupling. The fixed and movable portions generally lie in a plane. The resilient coupling is designed to allow motion of the movable portion relative to the fixed portion in response to acceleration along an axis of acceleration perpendicular to the plane and to resiliently restore the two portions to an equilibrium position when the acceleration ceases. One of the fixed and moveable portions of the silicon structure is electrically connected as a first electrode. The other of the fixed and moveable portions bears an electrically-conductive layer electrically connected as a second electrode. The first and second electrodes are arranged in capacitive opposition to each other. Electronics and/or software are designed to translate a measurement of capacitance between the first and second electrodes into a measurement of acceleration along the axis.
In general, in a third aspect, the invention features an accelerometer. A silicon wafer is etched to form a fixed portion, a movable portion, and a resilient coupling between. The fixed and movable portions generally lie in a plane. The resilient coupling is designed to allow motion of movable portion relative to the fixed portion perpendicular to the wafer in response to acceleration perpendicular to the wafer and to resiliently restore the two portions to an equilibrium position when the acceleration ceases. The mass of the movable portion is concentrated on one side of the resilient coupling. The fixed and moveable portions each bear an electrode, the electrodes being arranged in capacitive opposition. Electronics and/or software are designed to translate a measurement of capacitance between the first and second electrodes into a measurement of acceleration perpendicular to the wafer.
In general, in a fourth aspect, the invention features an accelerometer. A first electrode is oriented parallel to an axis of acceleration. A second electrode is oriented parallel to the axis of acceleration. A third electrode is coplanar with the second electrode. The second and third electrodes are arranged in capacitive opposition to the first electrode. A resilient coupling is designed to allow motion of the first electrode relative to the second and third electrodes along the axis of acceleration in response to acceleration and to resiliently restore the first electrode to an equilibrium position when the acceleration ceases. The second electrode is in opposition to a majority of the surface area of the first electrode when the electrodes are in the equilibrium position. Electronics and/or software are designed to translate a measurement of capacitance between the first and third electrodes into a measurement of acceleration along the axis.
Embodiments of the invention may include one or more of the following features. The fixed structure, movable structure and resilient coupling may be integrally formed primarily by etching a silicon wafer. The fixed structure and movable structure may be formed at least primarily of high aspect ratio beams. The third electrode may be connected to a ground potential. The first electrode may be formed as a high-aspect-ratio beam with a larger cross-sectional dimension oriented parallel to the axis of acceleration. Various structures of the movable and fixed structures may be electrically isolated from each other by isolation joints formed within the silicon wafer. Various structures etched from the wafer may be released from an underlying substrate of the silicon wafer. The electronics and/or software may measure differential capacitance between at least two pairs of electrodes, and translate the measured differential capacitance into an expression of acceleration. A capacitance between the fixed and movable electrode may be at a maximum when the movable structure is displaced from the equilibrium position. The resilient coupling may be a torsional flexure. The fixed and movable electrodes may be arranged in first and second regions, such that (a) motion in a direction of the movable structure results in increased capacitance between electrodes in the first region and decreased capacitance in the second region; and (b) motion in an opposite direction of the movable structure results in decreased capacitance between electrodes in the first region and increased capacitance in the second region. The axis of acceleration may be perpendicular to the wafer. The metal electrode may be formed as a layer deposited on the silicon of the movable portion. The movable portion may include a stop designed to engage a floor of the fixed portion to limit excess motion. The second silicon electrode may be in opposition to a majority of the surface area of the first silicon electrode when the electrodes are in the equilibrium position.
The above advantages and features are of representative embodiments only. It should be understood that they are not to be considered limitations on the invention as defined by the claims. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims.